I never seen someone intentionally injuring someone, but I've seen quite a few injures from Yudansha who think that throwing someone harder than usual means their aikido is better.

Personally, I've tweaked a couple of people during practice unintentionally, but I do fear that one day I will hurt someone seriously. I allow this thought to sometimes hinder my Aikido, especially around more fragile Ukes. Does anyone else worry about this, or have this problem?